Amyotrophic Lateral Sclerosis (ALS) is an invariably fatal disorder caused by degeneration of the upper and lower motor neuron. Presently, there are no effective therapies. Many promising interventions have failed in clinical trials. New therapies are needed and their development will depend on advances in our knowledge of mechanisms of motor neuron degeneration and of the support provided by the surrounding glia. Fused in sarcoma (FUS) is one of the latest genes discovered to cause familial and sporadic ALS. It is one of a growing number of RNA/DNA processing genes associated with familial and sporadic ALS. This discovery has further highlighted the pivotal role that defects in RNA/DNA function and processing have in neurodegeneration. A major unanswered question is whether loss of FUS function contributes to neuronal degeneration. Ours and other investigators studies support the notion that loss of nuclear function(s) plays a role in motor neuron degeneration, however, the importance of loss of function mechanism in vivo remains to be determined. Another critical question is whether neurodegeneration caused by FUS defects is cell-autonomous. Our studies in Drosophila suggest a glia-based mechanism of neurotoxicity. The hypothesis to be tested in this project is that loss of FUS function is a critical mechanism to cause motor neuron degeneration and non cell-autonomous FUS neurotoxicity mediated by oligodendrocytes plays an important role in ALS. Constitutive FUS knockout in mice resulted in neonatal lethality and male sterility, hence these mice cannot be used to investigate the role of loss of FUS in vivo in the adult nervous system. To fill this gap in knowledge, we will make FUS conditional knockout mice in which FUS is depleted in either motor neurons or oligodendrocytes and characterize age-dependent motor neuron loss and associated phenotypes. We have made a FUSfl/fl mouse line and we are ready to cross it with neuronal and oligodendrocyte Cre expressing lines. We will perform motor, pathological and molecular studies to determine whether FUS deficient motor neurons undergo age-dependent degeneration and whether FUS deficient oligodendrocytes contribute to motor neuron loss. Future studies will investigate the molecular mechanisms of disease in these two models. The ultimate goal is to generate models in which molecular targets can be identified, characterized and used to test novel therapeutic interventions for ALS. Collectively, these studies will fill an important gap in knowledge and will generate valuable mouse models that will be used to investigate disease mechanisms and serve as in vivo platforms to test compounds aimed at slowing progression of motor neuron loss. These studies have a great potential to generate results and reagents that will advance the field of ALS research and will benefit the health of Veterans with motor neuron disorders.